1. Field of the Invention
This invention relates to a radio resource management in the system which adopts CDMA as a multi-link method, more particularly, to a call attempt control method under the traffic overload of CDMA environment.
2. Description of the Related Art
A mobile station transmits messages (messages for call attempts, responses to the paging message etc.) on the access channel using the random access procedures. The parameters for the random access procedure are provided by access parameter messages which are transmitted on the paging channel from the base station.
As shown in FIG. 1, total procedure which transmits a message on the access channel and receives the ACK (acknowledgement) of the message is called as access attempt and each transmission in the access attempt is called as access probe. The mobile station transmits same messages to each access probe in the access attempt. Each access probe consists of access channel preamble and access channel message capsule as shown in FIG. 1.
Each access probe in the access attempt consists of access probe sequence and each access probe sequence consists of 1+NUM.sub.-- STEP access probes. The first access probe in each access probe sequence is transmitted to the starting portion of access channel as an average power level which is proportional to the normal open loop power level. EQU Mean TX Output Power(dBm)=-73-mean input power(dBm)+NOM.sub.-- PWR(dB)+INIT.sub.-- PWR(dB)
Succeeding access probes have more power of PWR.sub.-- STEP than preceding access probes and are transmitted. The messages transmitted on the access channel can be divided by two types, that is, the response messages to the messages of base station and the request messages which are transmitted voluntarily by the mobile station and different procedures are used to transmit the two messages. That is, in the request messages as shown in FIG. 1, the persistence test is performed for every slot before the access probe sequence is transmitted and the access probe sequence starts to be transmitted when the test ends successfully. Therefore, the random delay time of PD (Persistence Delay) occurs. The mobile station produces random number PR which number is between 0 and 1 to perform the persistence test and the test succeeds when the value of PR is smaller than the value P.
When the access channel request is for the message transmission and position registration, the value of P can be calculated according to the equation 1. ##EQU1##
When the access channel request is not for the message transmission and position registration, the value of P can be calculated according to the equation 2. ##EQU2##
n in the equations 1 and 2 represents overload level imposed in the mobile station.
When one access probe sequence ends, the persistence test starts after delay of RS which is backoff delay having random value between 0 and 1+BKOFF. Also, a random time delay occurs between one access probe and the next one in the access probe sequence. After the access probes were transmitted, the mobile station stays for TA=(2.times.ACC.sub.-- TMO).times.80 ms to receive ACK from the base station. The access attempt ends if ACK is received during that time. However, if ACK is not received during that time, the transmission starts after delay of RT which is another backoff delay having random value between 0 and 1+PROBE.sub.-- BKOFF slot.
In access attempt, a transmission timing point for an access a channel is determined by a PN randomization procedure, as shown in FIG. 1. For each access attempt, a mobile station calculates a delay time RN having value of a PN chip between zero(0) and 2.sup.PROBE.sbsp.--.sup.PN.sbsp.--.sbsp.RAN.sbsp.-- -1 by a hash function depending on ESN (electronic serial number) and then starts transmission after passing a delay time of the PN chip.
The access parameters which are transmitted to the paging channel from the base station of CDMA system for the random access procedure as follows.
TABLE 1 ______________________________________ parameter value ______________________________________ NUM.sub.-- STEP 6 NOM.sub.-- PWR 0 INIT.sub.-- PWR 0 PWR.sub.-- STEP 3 db PSIST(n), n=0, 1, - - - , 15 0 REG.sub.-- PSIST 0 MSG.sub.-- PSIST 0 BKOFF 1 ACC.sub.-- TMO 5 PROBE.sub.-- BKOFF 0 Max.sub.-- Req.sub.-- Seq 2 MAX.sub.-- CAP.sub.-- SIZE 0 PARM.sub.-- SZ 3 ______________________________________
The important matter for maintaining speech quality is to control unnecessary interferences to the minimum level since the capacity of the CDMA system can be determined by the interference. When the mobile station starts a new call in the state of traffic overload, the messages on the access channel transmitted using the random access procedure are not serviceable due to the error occurring on the transmitting channel. Accordingly, the mobile station which d id not receive ACK message to the transmission message retransmits the same message fixed times after desired time by the access procedure. The total power which the mobile station transmits becomes considerable as the number of mobile stations which require new calls increases, and the power acts as interference to the call which is on the line and then acts as elements interfering with the total speech quality.
In the conventional art, since it was determined that the transmitting power at the access attempt of mobile station caused a little effect on the system, the retransmitting effect was not considered due to the message failure transmitted on the access channel in the state of traffic overload. Therefore, the speech quality is not good in the state of traffic overload.